Converter Control with Virtual Admittance Under Current Limits
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Solution Overview
Problem
Power electronic devices in electricity networks face limitations in current capability, leading to disrupted grid forming capability under fault conditions, and it is difficult to predict system restoration post-fault.
Innovation Solution
A converter control system that includes a grid forming control with a current controller, a voltage controller emulating a physical synchronous machine, and a virtual admittance module to limit converter currents and stabilize the grid forming mode by controlling the angle and amplitude of the voltage reference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the converter current reference is limited to comply with converter capability, then the converter current capability is preserved, but the grid forming capability is disrupted
Solution Approach 1:
The virtual admittance parameter is dynamically adjusted based on the ratio between limited current and actual current. When current limiting is active, the virtual admittance is increased to maintain grid forming capability, and when current limiting is not active, the virtual admittance returns to its normal value. This dynamic adjustment allows the converter to maintain grid forming capability while respecting current limits.
Solution Approach 2:
A virtual admittance module is introduced as an intermediary between the current reference and the converter. This virtual admittance dynamically modifies the current reference based on the limiting status, acting as a mediator that preserves grid forming capability while ensuring current limits are not exceeded. The virtual admittance translates the current limitation into an equivalent impedance effect that maintains system stability.
2Reliability
If the converter operates under fault conditions with current limitation, then the converter is protected from overload, but the system restoration predictability deteriorates
Solution Approach 1:
The system continuously monitors the converter current and compares it with the current limit. Based on this feedback, the virtual admittance parameter is automatically adjusted. When the current exceeds the limit, the virtual admittance is increased to reduce the current reference, and when the current is within limits, the virtual admittance returns to normal. This feedback mechanism provides predictable system behavior during fault conditions.
Solution Approach 2:
The control system is designed to automatically detect fault conditions and activate current limitation with virtual admittance adjustment before damage occurs. The system preliminarily prepares the virtual admittance module to take over control when faults are detected, ensuring smooth transition and predictable restoration behavior without requiring complex post-fault analysis.
3Device complexity
If the voltage reference angle is not controlled during current limitation, then the control complexity is reduced, but the voltage angle may run away and grid forming capability is lost
Solution Approach 1:
The virtual admittance module acts as an intermediary that automatically adjusts based on current limiting status. When current limitation is active, the virtual admittance increases, which naturally stabilizes the voltage angle by modifying the current reference. This intermediary mechanism provides voltage angle stability during current limitation without requiring complex additional control algorithms.
Data Source
AI summary
A converter has a converter controller. Accordingly, the converter controller is configured to: provide a voltage reference having an amplitude and an angle, process the voltage reference to obtain a current reference to control a converter current, replace the current reference by a current reference limit if the current reference exceeds the current reference limit, and to control the angle of the voltage reference to minimize a difference between the current reference and the current reference limit if the current reference exceeds the current reference limit.